processes, sensitizers such as [Ru(bpy) 3 ]^2 þare acting as a one-
electron reagents, while many photocatalytically attractive pro-
cesses for the synthesis of permanent reaction products require
the exchange of more than one electron to avoid destructive side
reactions and thermodynamically unfavorable free radical chem-
istry (5,8). Serious efforts are therefore necessary to efficiently
couple the photoinduced primary processes to other catalytically
competent moieties. As an alternative, a new generation of light-
harvesting photocatalysts termedmultielectron transfer (MET)
photosensitizershas been introduced (8,111). These compounds
are able to directly promote net two-electron processes following
the absorption of only one photon.
Besides color, sensitization, and light-harvesting efficiency, the
actual reactivity of the excited states populated is therefore a
crucial property. Some aspects of relevance for bioinorganic and
biomimetic systems will be discussed in the following chapters.
B. PHOTOCHEMICALREACTIVITY
Absorption of light always leads to an activation of the
irradiated compound, as excited state energy levels are situated
considerably higher than those of the corresponding ground state
species. Thus, it is not surprising that many reactions, which are
thermodynamically or kinetically inaccessible in the ground
state, can occur with high efficiency from electronically excited
states. This general feature makes photochemical activation
FIG. 10. The photosensititzer [Ru(bpy) 32 þ]( 9 , left side) covalently
wired to the heme prosthetic group of horse heart apomyoglobin. This
hybrid system allows to create and characterize radical intermediates
in the enzyme active site triggered by light ( 110 ).
PHOTOSENSITIZATION AND PHOTOCATALYSIS 251